1. Field of the Invention
The present invention relates to a display data correction device, a display panel driver and display device using the same. More particularly, the present invention relates to a technique for correcting a grays-level value designated by display data by means of numerical operation for gamma correction or other purposes.
2. Description of the Related Art
Generally, a display panel driver driving a display panel, such as a liquid crystal display panel and a plasma display panel, is configured to provide gamma correction in accordance with the characteristics of the display panel. The gamma correction is a processing for displaying an image with desired brightnesses actually corresponding to gray-level values designated by display data. The display panel usually exhibits a non-linear association of brightness with the signal level of the drive signal (the drive voltage or drive current). For example, a voltage-to-transmission curve (V-T curve) of the liquid crystal panel is normally non-linear. This results in that an image cannot be displayed with desired brightnesses on the display panel when drive signals of signal levels proportional to the gray-level values designated by the display data are supplied to the display panel. The gamma correction is implemented so as to display an image on such a display panel with desired brightnesses correctly corresponding to the designated gray-level values.
The relationship between input data (that is, an input gray-level value) and output data for the gamma correction can be expressed by a gamma curve. The gamma curve is a curve specified in a coordinate system having a horizontal axis corresponding to the input gray-level value and a vertical axis corresponding to the output gray-level value (the gamma-corrected gray-level value). One issue of actual implementation of the gamma correction is architecture for achieving the gamma correction in accordance with a desired gamma curve.
Generally speaking, there are two known architectures for implementing the gamma correction. one known approach is to configure a driver circuit (typically, composed of D/A converters) which provides digital analog conversion for digital display data to thereby generate drive signals, so that the gray-level values nonlinearly correspond to the signal levels of the drive signals. In a controller driver driving a liquid crystal panel, for example, a gray-level voltage generation circuit that supplies gray-level voltages to the D/A converters is configured so that the voltage levels of the gray-level voltages increase nonlinearly to the gray-level values. The gamma correction is achieved by performing digital-to-analog conversion using the gray-level voltages generated nonlinearly to the gray-level values.
Another known approach is to incorporate an arithmetic circuit performing a numeric operation on display data. An advantage of the gamma correction by means of the numeric operation is the high flexibility in setting the gamma characteristics. Different display panels have different gamma characteristics, and installation environment causes an influence on the gamma characteristics. Therefore, high flexibility is desired in setting the gamma characteristics. The approach based on the numeric operation allows providing improved flexibility for setting the gamma characteristics, since the setting can be arbitrarily adjusted by changing parameters used in the numeric operation.
One of the most common techniques for achieving gamma correction based on arithmetic operation on display data is to use an LUT (lookup table) which describes an association of each allowed gray-level value of the input display data with the corresponding gray-level value of corrected display data. When an input display data is supplied, the LUT outputs the gray-level value of the corrected display data corresponding to the gray-level value of the input display data to achieve the gamma correction.
According to the inventors' study, gamma correction processing based on an LUT has two problems as follows. One problem is that the use of an LUT undesirably increases hardware unitization. When the input display data are 10-bit data and the corrected display data are 12-bit data, for example, it is necessary to describe data of 12×210×3 bits in the LUT; it should be noted that different gamma characteristics need to be set for different display colors. Another problem is that the gamma characteristics cannot be changed over instantly. When an LUT is used for gamma correction, it is required to rewrite the LUT to change over the gamma characteristics. Since it takes long time to rewrite the LUT, it is difficult to instantly change over the gamma characteristics by rewriting the LUT. In another approach, a plurality of LUTs may be prepared for eliminating the necessity of rewriting the LUT. However, this approach undesirably makes the problem of the increase in the hardware utilization more serious.
In this background, the inventers have been studying a technique for achieving gamma correction without using an LUT. Japanese Patent No. 4,086,868 B discloses a technique for achieving gamma correction based on quadric correction arithmetic expressions, which is also developed by the inventors. In this technique, parameters of the correction arithmetic expression are designated by correction point data. That is, the gamma characteristics are designated by the correction point data. The correction point data are defined as values that serve as indices of the gray-level values of corrected display data corresponding to the respective gray-level values of input display data. That is, modification of a value of a correction point data results in modification of the shape of the gamma curve in the vertical direction in a coordinate system in which the horizontal axis corresponds to the gray-level value of the input display data and the vertical axis corresponds to that of the output display data (or the gamma-corrected data). The technique of Japanese Patent No. 4,086,868 B is also designed to improve accuracy in gamma correction by changing over correction arithmetic expressions in response to the correction point data and the input display data.
The study of the inventors, however, there is room for improvement of accuracy in gamma correction and decrease in hardware utilization in the technique of Japanese Patent No. 4,086,868 B. According to the technique of Japanese Patent No. 4,086,868 B, the shape of the gamma curve specified in the coordinate system having the horizontal axis (horizontal direction) indicating the input gray-level value and the vertical axis (vertical direction) indicating the output gray-level value (gamma-corrected gray-level value) can be modified in the vertical direction by changing the correction point data but not modified in the horizontal direction. That is, the technique of Japanese Patent No. 4,086,868 B does not provide flexibility for control of the shape of the gamma curve. This may results in that the technique of Japanese Patent No. 4,086,868 B suffers from a limit in improvement of the accuracy of the gamma correction. Furthermore, the architecture disclosed in Japanese Patent No. 4,086,868 B in which the correction arithmetic expressions are changed over may disadvantageously cause an increase in hardware utilization.